Fluid containment vessels generally are fitted with pressure relief devices that will automatically vent the vessel to the atmosphere when a maximum permissible pressure level is reached within the vessel, thus preventing failure of the vessel. For example, barges used for transporting liquids have fluid containment hulls that are not designed to withstand pressures greater than about 3.0 psi. Consequently, it has long been the practice to provide safety vents on the barge, which will automatically open to the atmosphere when pressure of the fluid containment barge vessel reaches a maximum permissible pressure.
An example of one type of pressure relief device used on containment vessels can be found in U.S. Pat. No. 3,310,197. This device consists essentially of a rupture disk which is covered by a non-sealable but pivotably mounted cap to keep dirt and foreign material off the disk. If the disk is ruptured, the tank cannot be positively sealed without replacing the ruptured disk.
In U.S. Pat. No. 3,598,277, there is disclosed a safety relief valve that includes a rupture disk and a closure cap. The closure cap is mounted in such a way that it can move vertically upward, permitting communication of the tank with the atmosphere in the event of the failure of the rupture disk. The cap can move vertically downward so as to close the opening in the tank when the pressure in the tank is reduced to approximately atmospheric pressure. The disadvantage of such a device, however, is that it does not positively seal the tank contents from the atmosphere. Indeed, if the contents of the tank have a relatively high vapor pressure, the vapor would tend to lift the cap.
Other devices that have been employed as safety valves include spring loaded, and weighted covers for the tank openings. In some instances, these devices have also been-provided with bolting devices for positively sealing the cover when the containment vessel is loaded. The disadvantage of these types of devices, of course, is that the cover may inadvertently be secured prior to filling of the container, thereby preventing the assembly from relieving pressure, if necessary, with the result that the safety feature of the assembly is lost.
Accordingly, there remains a need for an improved rupture disk assembly that will prevent excessive pressure from developing in liquid containment vessels, especially during closed loading service, which will possess self-closing ability after rupture, thereby automatically sealing the tank contents from the atmosphere. Finally, also, there is a need for a rupture disk assembly that can be positively sealed but which will not readily lend itself to being inadvertently secured.